When a rubber or plastic insulated cable having a stranded conductor is submerged, water tends to propagate from an end, a joint portion, or a damaged sheath thereof into a longitudinal direction thereof, and consequently, the cable gets badly deteriorated. To overcome this problem, a water-tight rubber or plastic insulated cable has been proposed. This cable comprises a conductor of stranded wires, a water-tight compound, and a rubber or plastic insulation layer. The apertures among these stranded wires are filled with the water-tight compound, and the stranded wires are covered with the rubber or plastic insulation layer. In the manufacture of the cable, when the wires are stranded with each other, the water-tight compound is continuously extruded into the apertures by means of an extruder. Then, the water-tight compound is cured, and, thereafter, the conductor and the water-tight compound are covered by the rubber or plastic insulation layer. Here, inner and outer semi-conductive layers, a sheath, etc. are not explained.
In this water-tight rubber or plastic insulated cable, there are some relations between the viscosity of the water-tight compound and the water-tight characteristic of the completed cable. That is, the lower the viscosity of the water-tight compound is, the easier it is for the water-tight compound to be continuously extruded into the apertures. On the other hand, the water-tight compound of the completed cable must be properly stiff in its viscosity to provide the water-tightness thereof. Moreover, the water-tight compound must be stiff at a pre-heating stage just before the rubber or plastic layer extrusion process, in case of the conductor has the water-tight compound over the conductor surface, as shown in FIG. 1.
Taking the above relations into consideration, the water-tight rubber or plastic insulated cable disclosed in Japanese Patent Publication No. 63-25447 has been proposed by this applicant. According to this publication, the water-tight compound has a Mooney viscosity of 15 to 25 under a temperature of 130.degree. C. In addition, the stranded conductor is compressed to have a solidity factor of 85 to 95%. The water-tight compound is injected into the apertures except for the outermost surface of the stranded conductor. From the introduction of the compressed conductor and the definition of the viscosity of the compound, the continuous extruding operation thereof becomes easier and the water-tight characteristic thereof becomes more improved than the conventional cable.
However, even the above improved water-tight rubber or plastic insulated cable has disadvantages in that continuous operation of extruding water-tight compound into apertures of the stranded conductor is limited to two or three days, since the flow of the water-tight compound becomes deteriorated in those days due to the curing of the water-tight compound in the extruder, and that the water-tight characteristic is not obtained as expected only by the definition of ,oney viscosity.